Structures are known wherein seat rails are attached to brackets respectively fixed to an inner panel and a floor panel that configure a rocker (see for example Japanese Patent Application Laid-Open (JP-A) No. 2007-050784). Structures are also known wherein seat rails are fixed to faces of a side sill and a tunnel that face each other along the vehicle width direction (see for example JP-A No. 2006-123731). Moreover, structures are known wherein vehicle width direction outside joining portions between a side sill inner and a side sill outer are configured so as to be aligned in an up-down direction (see for example JP-A No. H05-208688). Structures are also known wherein vehicle width direction inside joining portions between a side sill inner and a side sill outer are configured so as to be aligned in an up-down direction (see for example Japanese Utility Model Application Laid-Open (JU-A) No. S61-125853).